1. Field of the Invention
This invention relates, in general, to methods and apparatus for offshore oil and gas production, and in particular, to a buoyancy can for tensioning, or supporting, the upper end of an offshore oil and gas riser that can be coupled to and decoupled from the riser without disassembling the upper terminal end portion thereof.
2. Related Art
Top-tensioned riser (“TTR”) systems for offshore oil and gas production (see, e.g., U.S. Pat. No. 4,702,321 to E. E. Horton) use passive “buoyancy cans” to support the risers independently of an associated floating production platform. In such a system, the riser extends vertically upward from the sea floor through the keel of the platform, and thence, to the well deck thereof, where it connects to a “stem” pipe, to which the buoyancy can is attached. The stem pipe extends vertically upward through an axial bore in the can and exits through its upper surface, where it may support a “work platform” to which the riser and its associated surface tree or “goose neck” are attached. A flexible, high pressure jumper then connects the outlet of the surface tree or goose neck to the production deck of the platform.
By comparison, a “hybrid” riser system typically comprises three main parts: A foundation anchor and flow-line interface unit, a multi-bore riser string, and a top end buoyancy can, which also carries the respective interfaces for the flexible jumpers, and which may be deployed on either the surface of the water or submerged below it. In such systems, the riser string is fabricated onshore as a complete, single-piece unit for tow-out and installation with a minimum of offshore work. The flexible jumpers are installed separately as part of the commissioning work, and the flow-lines are pulled in to the platform, which is outfitted with standard “hang-off” porches.
In either case, since the riser is independently tensioned, or supported, by the buoyancy can relative to the production platform, the platform can move relative to the riser, and indeed, may even temporarily depart from the production location, such that the riser is thereby independent of and isolated from the motions of the platform. However, in such an arrangement, the buoyancy can must have sufficient buoyancy to provide the required top tension in the riser, as well as support for the weight of the can, the stem pipe and at least part of the weight of the jumpers.
When a buoyancy can is initially deployed on a riser, or alternatively, when a deployed can is replaced with another can for repair or maintenance reasons, it is necessary to temporarily support the riser at a point below the can, and to remove the upper end, or terminal, portion of the riser, including the tree and any goose neck thereon, so that the “old” can, if any, may be slid up and off of the riser, and the “new” can may be slid down and over the riser. The upper terminal end portion of the riser must then be replaced and coupled to the new can for support. This results in a fairly complex, time-consuming, expensive, and potentially risky operation, particularly if effected in moderate or heavy seas.
A long felt but as yet unsatisfied need therefore exists for a buoyancy can that can be coupled to and decoupled from a riser either on or below the surface of the water without the need for removing the upper terminal end portion of the riser.